In order to maintain a more appealing shopping environment, supermarkets display some merchandise in open display cases that are refrigerated without doors.

While open display cases allow the produce to be more salable, the absence of any case doors presents some additional concerns for a service technician. Understanding the unique operating principles of these cases is needed when troubleshooting problems associated with them.

There are several types of open display cases, but they can be categorized into two general types: single-deck and multi-deck cases.

Single-deck cases are refrigerated cabinets in which the product is merchandized horizontally. Because some models resemble coffins, they are sometimes referred to as "coffin cases."

Multi-deck display cases merchandise the product in a layered, vertical fashion. This allows the product to be at eye level, making it more appealing to the customer.

In place of doors in open display cases, an air curtain is used to provide the needed barrier between the product and the store environment.

Air Curtains

Single-deck cases are designed to provide an air curtain across the top of the case. These cases are less prone to problems, since the colder air, which is heavier than the warmer store air, seems to stay in the case and not spill out.

Multi-deck cases are a little different. The air curtain is directed down the front of the case opening with the discharge air on the top of the opening and return air at the bottom. These cases are more prone to problems since it is easy for the air curtain to become disturbed. Most service problems related to the air curtain are found on the multi-deck cases.

Both low and medium temperature single-deck cases, as well as medium temperature multi-deck cases, usually have a single air curtain. Low temperature multi-deck cases usually have three separate air curtains. Since these cases are more prone to problems with heat leakage into the case, three air curtains are needed to provide a sufficient barrier: primary, secondary, and ambient.

The primary air curtain is the coldest air curtain and is closest to the product. The secondary air curtain is slightly warmer than the primary and is in front of it. The ambient air curtain is actually a blanket of store air that is directed straight down the front of the case. This air curtain is not circulated within the case.

Air curtains cannot totally eliminate all of the store air from migrating into a case. Small amounts always migrate through the air curtains into the case. Because of this, it is important to limit the store humidity level to 55 percent. High store humidity can cause the case's evaporator to frost up more quickly, resulting in frost accumulating on the product and the system, causing it to consume more energy. A 10 degree F increase in the store wet bulb temperature can increase the compressor's power usage by 25 percent.

Another major difference with open display cases is that they are designed and engineered to hold only the product at the temperature at which it enters the case. They are not designed to reduce a product's temperature. The product should be placed in the case close to its design holding temperature.

Open display cases are designed to primarily handle heat leakage into the case. Also, they are not generally designed to store product for a long period of time. Because of this, most case manufacturers recommend rotating product on a regular basis.

Servicing open display cases can present a challenge to any technician; however, understanding some of these basic operating principles should allow for more efficient troubleshooting.

Joe Marchese is owner of Coldtronics, Pittsburgh. He can be reached at 412-734-4433,, or

Publication date: 05/03/2004